Planetary aspects Occultation
Occultation
A blocking timed to the second
A nearer body passes in front of a more distant one and briefly hides it.
- Commonest kind
- The Moon occulting a star or a planet
- Disappearance
- Practically instantaneous
- Visibility track
- Usually tens to hundreds of kilometres wide
01 / Overview
Meet Occultation
An occultation is a nearer body passing in front of a more distant one and blocking it. The commonest is lunar: the Moon moves about 13° east along its orbit each day, switching background stars off and on as it goes. Asteroid occultations of stars are far rarer, yet they are one of the most effective ways to measure an asteroid’s shape.
An edge with no atmosphere
The Moon has no atmosphere, so an occulted star vanishes instantly, with almost no fading. That abruptness makes the event a good test for duplicity: if the light drops in two steps, the star is really a close pair.
Measuring a body by its shadow
When observers at different sites time the same asteroid occultation, their chords can be assembled into an outline of the asteroid. In 1977 astronomers watching Uranus occult a star discovered, unexpectedly, that Uranus has rings.
02 / Key data
Core parameters
- Commonest kind
- The Moon occulting a star or a planet
- Disappearance
- Practically instantaneous
- Visibility track
- Usually tens to hundreds of kilometres wide
- A landmark result
- The rings of Uranus, found in 1977
- Recurrence
- Lunar occultations happen every month
Data and calculation notes
Active periods, peak dates and rates are multi-year averages; each year differs a little, so plan against the forecast issued for the year in question.
03 / Observing tips
How to watch
- An occultation is visible only within a narrow track; check whether your site lies inside it.
- Timings need to be good to the second; a phone’s network clock is enough for amateur work.
- When the Moon occults a planet its disc is covered gradually over a second or two, unlike a star.
04 / Sources